I've given up with showers. If I can find the make I refer the customers to the manufacturers and if I can't find the make I suggest a new shower.
My experience generally involves an initial visit (cost).
Then time to find a replacement part (more cost).
Then a phone call, which often results with a "I'll come back to you."
Then a few weeks later "You came out to look at our shower, can you buy the cartridge?"
Then a scrabble through my notebook and if I can't find the necessary bits, another visit.
Then buy the part, fighting off the screams of "How much?"
Book customer in and visit with cartridge. Then spend ages trying to work out how to remove old part and find new part might require another O ring or something, let alone the difficulty of finding out how to isolate the shower without turning off the house water and draining a 3 mile long pipe run.
Then fit new cartridge and find it drips.
Cost of cartridge plus a good two hours of work results in £225? I can't charge that much.
Call to manufacturer will result it cost to customer of around £90 plus part. Or new shower might result in a two hour session and a £300 sparkly, new shower .
As for electric showers I undo cover, look for pipe and wire and ease of replacement and scorch mark, then replacement a few days later.